This invention involves a new, unconventional, mold-binding cast material to be used in orthopedic departments for the correction of deformities, fixation of fractured limbs and joints, and mollification of affected parts. The material is prepared by coating a stable, water hardening polyurethane prepolymer on a substrate of meshed synthetic or glass side chain fabric, characterized in that the stabilizer in the polyurethane prepolymer is maleic anhydride, and the catalyst is a mixture of bis(2-morpholinoethoxy)ethane and anhydrous potassium carbonate.
In recent years, U.S. Pat. No. 4,376,438 first proposed replacement of plaster casts in fracture treatment with a cast material formed by impregnation of fabric with a water hardening polyurethane prepolymer. Such a cast is used in the same way as the plaster cast, i.e., it is bound up on the patient's limb directly after impregnation with water. The water causes the prepolymer to polymerize, thereby producing a hardening and shaping effect on the relevant part of the patient. Since such cast material has the merits of being lightweight, high strength, fine porosity, water resistance, and X-ray penetrability, as well as convenience in use, it has found extensive application in medical and health institutions in various countries of the world. For the cast to harden quickly in 15 minutes as required clinically, it is necessary to incorporate a proper amount of catalyst in the polyurethane prepolymer. The presence of catalyst in the prepolymer and the invasion of little amount of moisture in the process of cast preparation and packaging may lead to the side reaction of gelatination of polyurethane prepolymer in the cast prior to use, or in the storage period, causing premature hardening of the cast, and thereby making it impossible to use.
To prevent initial hardening of the cast material, and improve the stability of the cast during storage, U.S. Pat. Nos. 4,433,680 and 4,644,280, as well as the Chinese patent 89103603.2, have proposed incorporation of benzoyl chloride into the polyurethane prepolymer as a stabilizing agent. However, benzoyl chloride is a highly corrosive and easily decomposing liquid. Even the addition of a small amount results in an irritating smell in the cast material, so that as a medical raw material, there will be difficulty in its being accepted by medical workers.
U.S. Pat. No. 427,002 proposes use of organic acids, such as phosphoric acid, as the stabilizer. Nevertheless, phosphoric acid mixed into prepolymer is liable to decompose into red phosphate, thus failing to have a stabilizing effect.
Both U.S. Pat. No. 4,574,793 and Chinese patent 90102934.3 have proposed the addition of a small amount of methyl sulfonic acid in the polyurethane prepolymer as a stabilizing agent. However, methyl sulfonic acid is a strong acid; once gaining entrance during preparation of polyurethane prepolymer, it will combine quickly with the OH group in the polyether, and reduce the stability of the prepolymer.
PCT/GB8700716 (W088/02636) proposes use of organic acid anhydrides, such as butanedioic anhydride, as the stabilizer. It is true that addition of it during preparation of polyurethane prepolymer may have a stabilizing effect, but butanedioic anhydride is incorporated in the prepolymer as solid particles by grinding into fine granules smaller than 75 .mu.m. The impossibility of a fairly even distribution affects stabilization of the prepolymer.